Evolution and Adaptation

by Thomas Hunt Morgan

If all this be granted, it is once more evident that the only variations that come under the action of selection are the limited number that are of vital importance to the organism. How little the theory of natural selection can be used to explain the origin of species will be apparent from the above quotation. This is, of course, not an argument against the theory itself, which would still be one of vast importance if it explained adaptive characters alone; but enough has been said, I think, to show that it is improbable that the origin of adaptive and non-adaptive characters are to be explained by entirely different principles.

In reply to a criticism of Mivart, Darwin makes the further admission as to the insufficiency of the theory of natural selection: “When discussing special cases, Mr. Mivart passes over the effects of the increased use and disuse of parts, which I have always maintained to be highly important, and have treated in my ‘Variation under Domestication’ at greater length than, as I believe, any other writer. He likewise often assumes that I attribute nothing to variation, independent of natural selection, whereas in the work just referred to I have collected a greater number of well-established cases than is to be found in any other work known to me.” If this is admitted, and if it can be shown that the evidence in favor of the inheritance of acquired characters is very doubtful at best, may we not conclude that Mivart’s criticisms have sometimes hit the mark?

The following objection appears to be a veritable stumbling-block to the theory. Flatfishes and soles lie on one side, and do not stand in a vertical position as do other fish. Some species lie on one side and some on the other, and some species contain both right-sided and left-sided individuals. In connection with this unusual habit we find a striking change in the structure. The eye that would be on the under side has shifted, so that it has come to lie on the upper side of the head, i.e. both eyes lie on the same side,—a condition found in no other vertebrate. As a result of the shifting of the eye, the bones of the skull have also become profoundly modified. The young fish that emerge from the egg swim at first upright, as do ordinary fish, and only after they have led a free existence for some time do they turn to one side and sink to the bottom. Unless the under eye moved to the upper side it would be of no use to the flatfish, and might even be a source of injury. Mivart points out that a sudden, spontaneous transformation in the position of eye is hardly conceivable, and to this Darwin, of course, assents. Mivart adds: “If the transit was gradual, then how such transit of one eye a minute fraction of the journey towards the other side of the head could benefit the individual is, indeed, far from clear. It seems even that such an incipient transformation must rather have been injurious.” Darwin’s reply is characteristic:—

“We thus see that the first stages of the transit of the eye from one side of the head to the other, which Mr. Mivart considers would be injurious, may be attributed to the habit, no doubt beneficial to the individual and to the species, of endeavoring to look upwards with both eyes, whilst resting on one side at the bottom. We may also attribute to the inherited effects of use the fact of the mouth in several kinds of flatfish being bent towards the lower surface, with the jaw-bones stronger and more effective on this, the eyeless side of the head, than on the other side, for the sake, as Dr. Traquair supposes, of feeding with ease on the ground. Disuse, on the other hand, will account for the less developed condition of the whole inferior half of the body, including the lateral fins; though Yarrell thinks that the reduced size of these fins is advantageous to the fish, as ‘there is so much less room for their action, than with the larger fins above.’ Perhaps the lesser number of teeth in the proportion of four to seven in the upper halves of the two jaws of the plaice, to twenty-five to thirty in the lower halves, may likewise be accounted for by disuse. From the colorless state of the ventral surface of most fishes and of many other animals, we may reasonably suppose that the absence of color in flatfish on the side, whether it be the right or left, which is undermost, is due to the exclusion of light.”

By falling back on the theory of inheritance of acquired characters Darwin tacitly admits the incompetence of natural selection to explain the evolution of the flatfish. If the latter theory prove incorrect, it must then be admitted that the evolution of the flatfishes cannot be accounted for by either of the two main theories on which Darwin relies.

Mivart further points out that the beginning stages of the mammary glands cannot be explained by Darwin’s theory. To which Darwin replies, that an American naturalist, Mr. Lockwood, believes from what he has seen of the development of the young of the pipe-fish (Hippocampus) that “they are nourished by a secretion from the cutaneous glands of the sac” in which the young are enclosed. This can scarcely be said to be a satisfactory reply; for, if it is true that this is the case for the pipe-fish,—and I cannot find on inquiry that this statement has been confirmed,—it is still rather speculative to suppose that the ancestral mammals nourished their young by secreting a fluid into the marsupial sac around the embryos.

Darwin deals with instincts of animals in the same way as he deals with their structures. After pointing out that instincts are variable, and that the variations are hereditary, he proceeds to show how selection may act by picking out those individuals possessing the more favorable instincts. In other words, the theory of natural selection is applied to functions, as well as to structure. Darwin makes use here also of the Lamarckian factor of inheritance, and concludes that “in most cases habit and selection have probably both occurred.”

A few examples will sufficiently serve to illustrate Darwin’s meaning. The first case given is that of the cuckoo, which lays its eggs in the nests of other birds, where they are hatched and the young reared by their foster-parents. The starting-point for such a perversion of the ordinary habits of birds is to be found, he thinks, in the occasional deposition of eggs in the nests of other birds, which has at times been observed for a number of species. For instance, this has been seen in the American cuckoo, which ordinarily builds a nest of its own. It is recorded and believed to be true that the young English cuckoo, when only two or three days old, ejects from the nest the offspring of its foster-parents, and this “strange and odious instinct” is supposed by Darwin to have been acquired in order that the young cuckoo might get more food, and that the young bird has acquired during successive generations the strength and structure necessary for the work of ejection. This is of course largely speculative, and it is by no means obvious that it was a greater benefit to the cuckoo to have other birds rear its young than to do so itself. We can equally well imagine, since this is the turn the argument takes, that the occasional instinct to deposit eggs in the nests of other birds would be disadvantageous, and could not have been acquired by the selection of a fluctuating instinct of this sort. We have no right to assume, that because a new habit has been acquired, that it is a more advantageous one than the one that has been lost. All that we can legitimately infer is, that, although the normal instinct has been changed into another, the race has still been able to remain in existence. The same conclusion applies to the case of Molothrus bonariensis, cited by Darwin, and is here even more obvious:—

“Some species of Molothrus, a widely distinct genus of American birds, allied to our starlings, have parasitic habits like those of the cuckoo; and the species present an interesting gradation in the perfection of their instincts. The sexes of Molothrus badius are stated by an excellent observer, Mr. Hudson, sometimes to live promiscuously together in flocks, and sometimes to pair. They either build a nest of their own, or seize on one belonging to some other bird, occasionally throwing out the nestlings of the stranger. They either lay their eggs in the nest thus appropriated, or oddly enough build one for themselves on the top of it. They usually sit on their own eggs and rear their own young; but Mr. Hudson says it is probable that they are occasionally parasitic, for he has seen the young of this species following old birds of a distinct kind and clamoring to be fed by them. The parasitic habits of another species of Molothrus, the M. bonariensis, are much more highly developed than those of the last, but are still far from perfect. This bird, as far as is known, invariably lays its eggs in the nest of strangers; but it is remarkable that several together sometimes commence to build an irregular untidy nest of their own, placed in singularly ill-adapted situations, as on the leaves of a large thistle. They never, however, as far as Mr. Hudson has ascertained, complete a nest for themselves. They often lay so many eggs—from fifteen to twenty—in the same foster-nest, that few or none can possibly be hatched. They have, moreover, the extraordinary habit of pecking holes in the eggs, whether of their own species or of their foster-parents, which they find in the appropriated nests. They drop also many eggs on the bare ground, which are thus wasted.”

Can we possibly be expected to believe that it has been to the advantage of this species to give up its original regular method of incubating its own eggs, and acquire such a haphazard, new method? Does not the explanation prove too much, rather than give support to Darwin’s hypothesis? Is it not better to conclude, that despite the disadvantages entailed by a change in the original instincts, the species is still able to remain in existence?